The present invention was conceived and developed under a U.S. Government Contract awarded by the U.S. Department of Energy. The Government has rights in this invention.
1. Field of the Invention
The present invention relates generally to methods for coating various substrates. More particularly, the present invention discloses a method for coating a substrate with a graphite powder to produce an article exhibiting low reflectance high emissivity at temperatures 1800xc2x0 F. or more.
2. Description of the Prior Art
Significant efforts have been directed to modifying the properties of known or existing materials in a manner that renders the materials suitable for use in particular applications. For example, it is desirable that various substrates in thermophotovoltaic power systems have surfaces with a stable high emissivity (xcex5 greater than 0.9) and low reflectance that may be exposed to temperature in excess of 1800xc2x0 F. As is well known to those skilled in the art, emissivity is the ratio of the radiant energy emitted by a surface to that emitted by a blackbody at the same temperature. Reflectance is the fraction of the total radiant flux incident upon a surface that is reflected and that varies according to the wavelength distribution of the incident radiation (Merriam-Webster""s Collegiate(copyright) Dictionary). The substrates may be made of a number of different materials such as boron nitride, silicon carbide, and various high temperature metal alloys. Typical emissivity values (xcex5) of the uncoated surfaces of these materials measured at 1.55 microns using a Pyrolaser(copyright) pyrometer, are in the range of 0.40 to 0.75.
In particular, a boron nitride substrate with the desired emissivity xcex5 greater than 0.9 is not currently commercially available. However, in view of the recent developments in the thermophotovoltaic power systems technology, there is a need for the stable high emissivity and low reflectance coating on the boron nitride substrate that can handle high temperature (around 1800xc2x0 F.) and remain intact.
Attempts to adhere a variety of different powders in various paste type mixtures, using various liquids and other physical forms, to the surface of the boron nitride substrate using paintbrush type techniques, resulted in obtaining surfaces with high enough emissivity (in the range from 0.8 to 0.9), but poor adhesion and uniformity. There were attempts to apply other materials, such as milk and peanut oil, to the boron nitride substrate using brush or spray techniques and expose them to high temperature (in excess of 2000xc2x0 F.). These substances produced dark colorations consistent with high emissivity, but the coating was not uniform.
Also, commercially available high temperature paints and other high temperature coating materials including Neolube (a solution of colloidal carbon in isopropanol from Huron Industries, Fort Huron, Mich.) were applied using brush or spray techniques to the surface of the boron nitride substrate. The resulting coating provided high emissivity (in the range from 0.8 to 0.9), but the surface of the substrate lacked proper adhesion and uniformity either before or, in some cases, after exposure to high temperature ( greater than 1800xc2x0 F.). The exception was the Neolube coating material that adhered well, but had low emissivity (approximately 0.7), which is not sufficient for the intended application.
Furthermore, carbon, graphite powder and other dark powders were mixed with the liquid coatings in order to further increase the emissivity of the surface of the substrate, and applied (using brush or spray techniques) as a coating to the boron nitride substrate. As a result, some surfaces with the emissivity in excess of 0.9 were obtained, but lacked proper adhesion and uniformity.
Therefore, there is the need for a method for applying a coating to a boron nitride substrate that provides stable high emissivity and low reflectance and can be exposed to high temperatures (around 1800xc2x0 F.).
An object of the present invention is to provide a graphite powder coating to various substrates, such as boron nitride, silicon carbide and various high temperature metal alloys, having high emissivity (xcex5 greater than 0.9) and low reflectance that may be exposed to high temperatures (around 1800xc2x0 F.).
The present invention discloses a novel method for applying a graphite powder coating to a substrate in order to provide a surface with high emissivity (xcex5 greater than 0.9) and low reflectance characteristics that may be exposed to high temperatures (around 1800xc2x0 F.). The present invention is particularly applicable to a substrate made of materials such as boron nitride, silicon carbide and various high temperature metal alloys. The present invention exhibits the advantages of having excellent bonding coupled with a desirable surface grain finish that can withstand high temperature requirements while maintaining good adhesion with a minimal amount of outgassing.
According to the present invention, a mixture consisting substantially of 75% by weight of a solution of colloidal carbon in isopropanol (preferably, Neolube from Huron Industries or the like) and 25% by weight of butyl alcohol is provided. First, a first layer of the mixture is applied to the surface of the substrate that has been cleaned and is void of oil and undesirable contaminants. After the mixture applied to the surface of the substrate dries, a second layer of the mixture is applied to the surface of the substrate that has been cleaned and is void of oil and undesirable contaminants. Then, a layer of the graphite powder is applied to the substrate over the second layer of the mixture while it is still wet.
Further, according to the present invention, an additional layer of the graphite powder may be applied over the previously coated surface or over any portion of the surface not coated in the first application in a manner disclosed above.
Therefore, the method for applying a graphite powder coating to a substrate in accordance with the present invention provides the substrate with a surface having high emissivity (xcex5 greater than 0.9) and low reflectance characteristics that may be exposed to high temperatures (around 1800xc2x0 F.).